This invention relates to a surface treatment technique whereby metal is electroplated onto an anodic oxide coating formed on the surface of aluminum-based materials to give conductivity to said anodic oxide coating.
When aluminum-based materials consisting of aluminum or aluminum alloy are subjected to anodizing in a sulfuric acid bath or oxalic acid bath, as shown in FIG. 3(A), a porous anodic oxide coating 1 can be formed on its surface. Such an anodic oxide coating 1 has the function of increasing the weather resistance of aluminum-based materials 2, so this is used widely in a wide range of fields such as building materials and decorative products and the like.
In addition, as shown in FIG. 3(C), when metal 7 is electroplated onto the interior of the pore 3 of each cell 4, it is given conductivity so it can have new applications such as crack-resistant anti-static materials. However, a thick barrier layer 5 is formed in the bottom of the pores 3 in a porous anodic oxide coating 1 formed by conventional methods, so in order to electroplate metal 7 onto the interior of the pores 3 to give it conductivity, as shown in FIG. 3(b), it is necessary to remove the barrier layer 5 formed in the bottom of the pores 3 and then perform the electroplating process. In a conventional method used to remove this barrier layer 5, after anodizing is performed in a sulfuric acid bath or oxalic acid bath, the anodizing voltage in the same electrolyte bath or a different electrolyte bath is gradually lowered over a period of 15 to 20 minutes, thereby electrochemically dissolving the barrier layer 5 at the bottom of the pores 3 in the anodic oxide coating 1. In another method in use, after anodizing is performed in a sulfuric acid bath or oxalic acid bath, the power is turned off and the workpiece is left in the same electrolyte bath or a different electrolyte bath for a period of 15 to 30 minutes thereby chemically dissolving the barrier layer 5 at the bottom of the pores 3 in the anodic oxide coating 1. Moreover, both the latter method of dissolving the layer chemically and the former method of dissolving the layer electrochemically may also be used together to dissolve the barrier layer 5.
However, processing takes a long time in any of these conventional methods, so they have a problem in that their productivity is low. In addition, all of these processes require complicated and special techniques, so they have another problem in that their quality is unstable.
To this end, the object of this invention is to provide a surface treatment method whereby, without taking a long time in processing, an anodic oxide coating with no barrier layer or with a barrier layer so thin that it exhibits tunneling on the bottom of the pores is formed stably on the surface of aluminum-based materials and then metal is electroplated onto the interior of the pores in said anodic oxide coating to give said anodic oxide coating conductivity.
In order to achieve the above object, in the surface treatment method for aluminum-based materials according to the present invention, anodizing of an aluminum-based material consisting of aluminum or aluminum alloy is performed in an anodizing bath comprising nitrate ion together with at least one ion selected from among an organic acid ion or an inorganic acid ion able to form a porous anodic oxide coating, thereby forming a porous anodic oxide coating on the surface of said aluminum-based material, and then, electroplating of said aluminum-based material in performed in an electroplating bath comprising metal ion so that metal is electroplated from said electroplating bath into the pores in said porous anodic oxide coating, thereby giving said anodic oxide coating conductivity.
In the surface treatment method according to the present invention, the anodizing bath contains nitrate ion together with an organic acid ion or an inorganic acid ion able to form a porous anodic oxide coating, so by using this anodizing bath to perform the anodizing of the surface of aluminum-based materials, at the same time that the porous anodic oxide coating is being grown, the barrier layer is being dissolved at the bottom of the pores. For this reason, by the time that the anodizing process is complete, the barrier layer in the bottom of the pores in the porous anodic oxide coating is thin enough to exhibit tunneling or there is no barrier layer in the bottom of the pores. For this reason, even without performing current restoration or galvanic dissolution or other complicated barrier layer removal process in order to remove the barrier layer from the bottom of the pores in the porous anodic oxide coating, by simply performing the electroplating immediately it is possible to electroplate metal onto the interior of the pores in the anodic oxide coating to give the anodic oxide coating conductivity or other new functions.
In the present invention, as the aforementioned anodizing bath, it is possible to use a bath containing, for example, 100 g/l-300 g/l of sulfuric acid and 7 g/l-140 g/l of nitric acid or a nitrate. In addition, as the aforementioned anodizing bath, it is also possible to use a bath containing, for example, 100 g/l-300 g/l of sulfuric acid, 7 g/l-140 g/l of nitric acid and 10 g/l-100 g/l of a nitrate.
In the present invention, the anodizing is performed with the anodizing bath at a temperature of 0xc2x0 C.-30xc2x0 C. and at a current density of 0.5 A/dm2-5.0 A/dm2. Here, if the temperature of the anodizing bath is roughly 0xc2x0 C.-30xc2x0 C., the porous anodic oxide coating can be formed stably. In addition, if the temperature of the anodizing bath is roughly 0xc2x0 C.-5xc2x0 C., a hard porous anodic oxide coating can be formed.
In the present invention, it is preferable that an electroplating bath containing silver ion as the metal ion be used in the electroplating process, so that silver is electroplated into the pores of the porous anodic oxide coating. To this end, it is possible to use a bath containing, for example, 5 g/l-20 g/l of a silver salt and 10 g/l-20 g/l of a nitrate as the electroplating bath. With such a constitution, highly conductive silver will be electroplated into the pores of the anodic oxide coating, so an anodic oxide coating with a low surface resistance value can be formed. In addition, silver has antibacterial action, so it is possible to give the anodic oxide coating antibacterial properties.
In the present invention, the electroplating is performed with the electroplating bath at a temperature of 20xc2x0 C.-30xc2x0 C.
In the present invention, the surface resistance value of the anodic oxide coating can be controlled by the amount of silver electroplated into the pores of the porous anodic oxide coating.
In the present invention, after electroplating, the anodic oxide coating may be colored by electroplating additional metal within the pores of the anodic oxide coating. In addition, the anodic oxide coating may also be colored after electroplating by affixing organic dyes or organic pigments within the pores of the anodic oxide coating. With such a constitution, a design can be applied to the anodic oxide coating.
In the present invention, after the electroplating, it is preferable to seal the pores in the anodic oxide coating by performing water-vapor sealing, boiling-water sealing or low-temperature sealing. With such a constitution, it is possible to stabilize the metal or the like electroplated into the pores in the anodic oxide coating.